An amplifier's gain and phase delay transfer characteristics may be constant at low power levels. However, as the drive level is increased, the phase delay increases, and the rate of gain change decreases, up to that drive level where the amplifier saturates. This nonlinearity of the gain and phase delay with drive level caused distortion in the transmitted signal. The amount of distortion decreases as the output power level is backed off from saturation. In data transmission techniques such as quadrature amplitude modulation (QAM), data is encoded as the amplitude and phase of an RF carrier signal. Thus, the distortion caused by amplifier nonlinearity can directly lead to data errors.
Because of distortion, a system requiring a linear amplifier must operate a power amplifier well backed off from saturation. To achieve a lower distortion level for a specified output power, a larger amplifier must be used. However, there is a penalty for increased linearity: the power added efficiency of the amplifier decreases as one backs off from saturation. Because of this inherent tradeoff between linearity and efficiency, there is a large incentive to compensate or linearize the amplifier, so that it can operate in a linear fashion closer to its saturation level. This lowering of the amount of distortion allows the use of a smaller amplifier at a given output power level, to thereby increase the efficiency of the amplifier.
To date, attempts to linearize the gain and phase delay of amplifiers have fallen into four main categories: predistortion; feed forward; RF feedback; and modulation feedback. Predistortion treats an amplifier as a black box with a transfer function. By judicious choice of another black box in front of the amplifier, with opposing gain and phase delay vs. drive transfer characteristics, the nonlinearities in the amplifier can be cancelled out, leaving a linear system.
The predistortion technique suffers from three main problems. The choice of proper transfer curves often must be done by eye. Yet many modern applications requiring linear amplifiers need extremely low distortion levels, where the distortion is due to very small nonlinearities that cannot be discriminated by the eye. Each predistortion linearizer must be individually tuned, increasing production costs. Finally, although the compensation is unconditionally stable because it is open loop, it nevertheless cannot compensate for variations caused by age and environmental changes.
Feedforward linearization involves coupling off the input and output of the amplifier, then comparing the two signals directly at the carrier frequency. One then takes the error signal, amplifies it, and sums it back in with the output signal. As with predistortion, this is an open loop technique, unable to adapt. It also requires two power amplifiers, giving only a marginal increase in efficiency.
The third category of prior linearizers employs negative RF feedback to increase linearity. This dynamic technique is more tolerant to circuit and environmental changes, requires little or no production tuning, and usually gives better results. However, at RF and microwave frequencies, the dimensions of the loop become significant. Negative feedback requires that the feedback signal be 180.degree. out of phase with the input signal. However, if the circuit has a large delay, then phase variation with frequency can render the loop unstable. This means that the loop gain must have a very narrow bandwidth, limiting its usage to very narrow band systems, which are difficult to build, or to low frequency systems. This is particularly true for long amplifier chains or traveling wave tube amplifiers, where the loop is multiple wavelengths long.
Modulation feedback is a more recent technique that is similar to RF feedback. In many communications systems, the power amplifier is used as the transmitter, sending an information-bearing modulated carrier. Modulation feedback linearizes both the modulation process and the transmitting amplifier. It does this by demodulating a small coupled off portion of the output signal, and comparing it to the base band modulating signal. The difference can then be used to control the modulation process, giving both gain and phase delay compensation. The main drawback of this technique is that it cannot be used in a repeater, where the original modulating signal is unavailable.